The Clackamas River is an approximately tributary of the Willamette River in northwestern Oregon, in the United States. Draining an area of about , the Clackamas flows through mostly forested and rugged mountainous terrain in its upper reaches, and passes agricultural and urban areas in its lower third.
The river rises in eastern Marion County, about east-southeast of Salem. The headwaters are on the slopes of Olallie Butte in the Mount Hood National Forest, about north of Mount Jefferson, at an elevation of in the Cascade Range. The Clackamas flows briefly north and then flows northwest through the mountains, passing through North Fork Reservoir and Estacada. It then emerges from the mountains southeast of Portland. It joins the Willamette near Oregon City and forms the boundary between Oregon City and Gladstone.
The Clackamas provides hydroelectric power and drinking water for some of the Portland metropolitan area, and it supports runs of Coho salmon, spring and fall Chinook salmon, and summer and winter steelhead. The river's old-growth forests, its habitat for several species of birds, its healthy fish runs, and the recreational opportunities that it provides—such as fishing and whitewater rafting—led to the designation of more than half of the length of the river into the National Wild and Scenic Rivers System (NWSRS). This environment also allowed Native Americans to settle in the river's basin as early as 10,000 years ago.
Regulation of the river began in 1905 with the Cazadero Dam. In 1912, the River Mill Dam intercepted wood and coarse sediment. Later dams at North Fork, Oak Grove, Stone Creek, and Timothy Lake also intercepted wood sediment on the lower river.
Course
The Clackamas River arises on the western slopes of the Cascade Range near Olallie Butte, between Mount Hood and Mount Jefferson in the Mount Hood National Forest. Flowing generally northwest and then west for about , it joins the Willamette River at Gladstone. The river falls nearly between its source and its mouth.
Originating in Marion County, the Clackamas River receives Squirrel Creek from the left bank and Lemiti Creek from the right bank before entering Clackamas County about from the mouth. Over its next , much of which is in a relatively level stretch known as Big Bottom, the river receives Cub Creek from the left, Sisi Creek from the right, then Hunter, Fawn, Rhododendron, and Lowe creeks, all from the left, followed by Wall, Pinhead, and Campbell creeks, all from the right, Kansas Creek from the left, and Cabin Creek and Lost Creek, both from the right. About from the mouth, Granite Creek enters from the left, and the river flows by Austin Hot Springs and Picnic Area. Shortly thereafter, Switch Creek enters from the right, and at about from the mouth, the Clackamas receives the Collawash River from the left. At the confluence, Two Rivers Picnic Area is on the left and Riverford Campground is on the right. About further downstream, Trout Creek enters from the left, and Riverside Campground is on the right.
The Clackamas River supplies drinking water to more than 200,000 people. The City of Estacada, Clackamas River Water, the combined Oak Lodge Water District and Sunrise Water Authority, the South Fork Water Board, and the City of Lake Oswego all draw water from the Clackamas.
The five are opportunities for recreation such as white water rafting near the Portland metropolitan area; anadromous fish habitat supporting wild late winter coho, spring chinook, and winter steelhead; habitat for the federally threatened bald eagle and northern spotted owl; potential habitat for the threatened peregrine falcon; the forests of old-growth Douglas-fir along its banks; and historic importance. Another of the South Fork Clackamas River were designated as Wild and Scenic along with the creation of the Clackamas Wilderness in 2009.
Pollution
In March 2008, the United States Geological Survey (USGS) released a report entitled "Pesticide Occurrence and Distribution in the Lower Clackamas River Basin, Oregon, 2000–2005." It details pesticide pollution in the lower Clackamas River, its tributaries, and in pre- and post-treatment drinking water.
<blockquote>In all, 63 pesticide compounds: 33 herbicides, 15 insecticides, 6 fungicides, and 9 pesticide degradates were detected in samples collected during storm and nonstorm conditions. Fifty-seven pesticides or degradates were detected in the tributaries (mostly during storms), whereas fewer compounds (26) were detected in samples of source water from the lower mainstem Clackamas River, with fewest (15) occurring in drinking water.</blockquote>
The study concluded, "Given their frequent and widespread occurrence, especially during storms, pesticides have the potential to affect aquatic life and the quality of drinking water derived from the lower river," and laid out areas for further study.